The present invention relates to an anechoic chamber which is used in the measurement of the radiation noise field strength of equipment that generates radio noise causing radio interference to other equipment, and is used in cases where malfunctioning is tested by radiating radio waves of a high electromagnetic field to electronic apparatuses. More particularly, the present invention concerns an anechoic chamber in which the entire chamber is provided with electromagnetic shielding by metal plates, and which uses a multilayered wave absorber in which a structural member having a dielectric material provided on the rear surface of a wave absorbing member, such as a ferrite, is disposed on each metal plate.
A conventional multilayered wave absorber is disclosed in U.S. Pat. No. 4,972,191, wherein a ferrite is provided on an outermost surface facing oncoming radio waves, a dielectric material is provided on the entire rear surface of the ferrite, and a wave reflector is provided on the entire rear surface of the dielectric material.
The attachment of such a multilayered wave absorber to an interior surface of an anechoic chamber is generally effected by the use of an adhesive.
In the anechoic chamber which is required to have an electromagnetic shielding performance, as for the wave reflector which is a constituting element of the multilayered wave absorber, there are cases where the wave reflector is also used as a shielding material covering the entire chamber, and cases where the wave reflector is installed in a separated form on the chamber interior-side surface of the shielding material. Metal plates are generally used as the shielding material and the wave reflector.
In attaching the dielectric material to the wave reflector and attaching the ferrite to the dielectric material, an epoxy resin adhesive is generally used in the light of reliability. The distance between the rear surface of the ferrite and the surface of the wave reflector and both the types of material and thickness of the dielectric material and adhesive located therebetween constitute factors affecting the wave absorbing characteristic.
In addition, the ferrite of the multilayered wave absorber has a thickness of several millimeters in the light of its characteristic, and is in the form of a tile with longitudinal and transverse dimensions of 100 mm or thereabouts. Each tile of this ferrite is attached to the dielectric material at a site by using an adhesive.
As the electromagnetic shielding member serving as the wave reflector, the following structures are generally used: a structure in which thin iron sheets each having a thickness of 1 mm or less are plated, and the iron sheets are joined by soldering, a structure in which iron sheets each having a thickness of 1 mm or more are used, and the iron sheets are welded and are shield-joined, and a structure in which panels are formed by iron plates and shield-joined by metal fasteners.
When the anechoic chamber is constructed by using the multilayered wave absorber as described above, there are problems which are described below.
Even if a highly reliable adhesive is used in attaching the dielectric material, there is a possibility that an exfoliation may occur between the dielectric material and the wave reflector due to inadequate handling of the adhesive at the site or deterioration with age in the light of durability , resulting in the detachment of the dielectric material. A special plywood having a relatively large size with a longitudinal dimension of 1800mm and a transverse dimension of 900 mm may be used as the dielectric material.
As the dielectric plate which is attached to the electromagnetic shielding member serving as the wave reflector, special plywood having a thickness of 10 mm or thereabouts is used, and, as for the size of the plywood, the efficiency of execution of work is considered to be good if the plywood is used in a so-called standard 3.times.6 feet size (910 mm .times.1820 mm), a 4.times.8-feet size (1210 mm .times.2420mm), or a metric size (1000 mm .times.2000 mm) as it is.
In attaching the dielectric plate, the distance between the chamber interior-side surface of the shielding member and the dielectric plate needs to be set to an appropriate design value for obtaining the wave absorbing characteristic. If the dielectric plate is standard-sized, there are cases where a warp of the plate is 3 mm to 10 mm or more, and the thickness of an applied coat of the adhesive is 3 mm to 5 mm to ensure that the distance between the chamber interior-side surface of the shielding member and the dielectric plate is set to the design value. As a result, if the warp of the plate is large, there are cases where portions which are not bonded to the shielding member appear, causing the exfoliation of the dielectric plate.
To overcome the problems of faulty bonding and exfoliation which are ascribable to such warping of the dielectric plate, it is necessary to provide support which is called "temporary fixing" at a multiplicity of points until the adhesive is cured. Hence, there are problems in that time and trouble are required for the on-site execution of work, and that skilled operators are required.
The thickness (interval) between the rear surface of the ferrite and the surface of the wave reflector is determined by not only the thickness of the dielectric material but the thickness of the adhesive layer, so that ample care is needed in the amount of the adhesive applied, the applying method, and in the method of supporting the dielectric material. If the thickness of the adhesive layer exceeds an allowable range, there is a possibility of deterioration of the wave absorbing characteristic.
In addition to the method of the above in which the dielectric plate is bonded to the electromagnetic shielding member while supporting the dielectric plate by temporary fixing, a method is also commonly practiced in which the dielectric plate is attached by the sole use of metal fittings such as screws or by the joint use of metal fittings and an adhesive. However, a large number of screws or the like are required to overcome the problem of faulty bonding (the occurrence of portions which are not bonded to the shielding member) due to the warping of the standard-sized dielectric plate. In this case, a special measure is needed to attach the metal fittings such as screws to the shielding member. If the number of metal fittings such as screws becomes large, there is a problem in that the efficiency of execution of work declines, resulting in higher costs. In particular, in the case of the aforementioned shielded structure using thin iron sheets, each having a thickness of 1 mm or less, the problem becomes serious.
Furthermore, if the metal fittings such as screws are passed through the shielding member without taking an appropriate measure, there is the problem of deterioration of the electromagnetic shielding performance due to wave leakage through the gap between the metal fitting and the shielding material. For example, in the case of l mm gap, the shielding characteristic is deteriorated about 20 dB at 100MHz, and about 30 to 40 dB at 1000MHz.
There are problems in that the on-site execution time becomes prolonged since the ferrite is attached to the surface of the dielectric material at the site, and that operators who have a high level of skill are required for controlling the thickness of the adhesive.
If the ferrite wave absorbing members are attached to the standard-sized dielectric plate to form a wave absorber panel, and this wave absorber panel is attached to the electromagnetic shielding member, the weight of the wave absorber panel becomes 50 to 60 kg/panel, so that various problems arise in handling.